1. Field of the Invention
This invention relates to a photography lens which is adapted for imaging an object to be photographed and is capable of adjusting the magnification of a picture image.
2. Description of the Prior Art
The conventional lens of this type has been arranged as follows: The image of an object to be photographed is captured by a focusing lens group of the photograph taking lens. Then, within a cylindrical lens barrel, the focusing lens group is moved in the direction of the optical axis of the lens to adjust it to an in-focus position. In a photograph taking lens of this type, a focusing lens group is mounted on the lens barrel which is either fixedly attached to a camera or arranged to be attachable to and detachable from the camera through suitable coupling means such as a bayonet mount. A focusing ring is mounted on the outside of the lens barrel. Between the focusing ring and the lens holder is means for transmitting a rotatory motion and for converting it into a linear motion in the direction of the optical axis of the lens, such as a helicoidal mechanism. With this arrangement provided, a focusing operation is performed by adjusting the movement of the lens holder through an operation to rotate the focusing ring around the optical axis.
Devices called automatic focusing devices have been developed and have recently come to be used for cameras. The automatic focusing devices of the prior art are arranged to adjust the position of a focusing lens group by automatically moving it upon detection of the position of an object to be photographed. These automatic focusing devices include varied types such as the type using the principle of trigonometrical measurement, the contrast type utilizing the contrast of the image of an object formed on an imaging plane (or at a position equivalent to the imaging plane), etc. Most interchangeable lenses for single-lens reflex cameras are of the contrast type.
There have been proposed single-lens reflex cameras incorporating such automatic focusing devices with interchangeable lenses having such automatic focusing devices disposed within their lens barrels. Typical examples of these single-lens reflex cameras and interchangeable lenses have been disclosed, for example, in pending U.S. patent application Ser. No. 280,549, filed July 6, 1981, U.S. Pat. No. 3,972,056, etc.
In accordance with the arrangement of the prior art, a driving device for driving a lens holder of a lens barrel in response to a signal from an automatic focusing device is comprised of a motor for driving the lens holder in response to an automatic focusing signal, and a gear mechanism for transmitting the rotating force of the motor to the lens holder. However, since the components of the lens barrel are arranged separately from the motor and the gear mechanism components, the arrangement of the prior art results in numerous parts and many connection parts between one unit and another. This has presented problems in terms of the cost and difficulty of assembly and adjustment work.
U.S. Pat. No. 4,152,060, entitled "Epicyclic Electromechanically Actuated Lens Drive", discloses a device in which the stator element and the movable element of an electromagnetic induction mechanism are functionally incorporated within a lens barrel; and a lens holder which carries a focusing lens group is arranged to be driven and controlled along the optical axis by virtue of electromagnetic induction. In this case, the lens holder is rotatably attached to the inside of a cylindrical armature while a stator is arranged coaxially with the optical axis of the lens. The armature and the lens holder are connected to each other by a mechanical coupling. A stepwise rotatory motion of a rotating device is converted into a stepwise focusing movement of the lens holder which takes place along the optical axis. The mechanical coupling is effected through engagement of the gear teeth of the lens holder and the armature. In the device according to U.S. Pat. No. 4,152,060, the driving force on the lens holder in the optical direction must be obtained by converting the rotatory motion performed by the stator and the armature into a linear motion, resulting from the gear tooth coupling. This arrangement tends to have a loss in the efficiency of the driving force transmission. Besides, the use of an epicyclic motor also requires a high degree of precision in machine work on the parts involved in gear tooth coupling between the armature and the lens holder. It is a further shortcoming of this device that back-lash in the gear tooth coupling part causes some error in the movement of the lens holder and this problem also calls for a solution.
Further, in the field of zoom lenses which are arranged to vary the magnification of the picture image of an object to be photographed, lenses arranged to accomplish a position control over a variator lens group and a compensator lens by an electrical process are known, for example, U.S. Pat. Nos. 4,161,756 and 3,884,555, etc. The driving force for the lens holders of these variator and compensator lenses is arranged to be transmitted from a motor through a gear train mechanism. Therefore, the lens drive mechanism of these patents is clumsy or unshapely and has shortcomings with respect to the cost and assembly and adjustment work in the same way as in the case of the focusing lens arrangement mentioned in the foregoing.